1. Field of the Invention
This invention relates to telephone subscriber loop circuits and, more particularly, to a constant current line feed circuit for such loop circuits.
2. Description of the Prior Art
It has become increasingly common to serve telephone subscribers over carrier systems operating on the subscriber loops. This permits the transmission of a larger number of telephone conversations on a fewer number of metallic pairs. In some situations, such as long rural loops, the cost of additional metallic pairs can exceed the cost of carrier terminals and therefore the use of electronic carrier systems in the loop plant can be justified. One such carrier system is shown in the copending application of the present applicant Ser. No. 974,384, filed of even date herewith.
In such carrier derived loops, and in many standard telephone loops, it is desirable to maintain the level of direct current in the metallic connection to the subscriber (the local drop) at a preselected value regardless of the length of the metallic path. In a system that is power limited, such as one totally powered from the central office and limited due to the resistance of the transmission medium, it is imperative to insure that the power utilized by remote circuits is kept to a minimum within the constraints of the system. Therefore, instead of providing power to the local drop in such a way as to result in variable currents to the telephone set, as is the case with the standard constant voltage power feed circuits provided by local central offices, it becomes desirable to provide all telephone sets with the minimum current permissible, i.e., constant current power feed. In this manner the current provided becomes essentially independent of the impedance, and hence length of the local drop, as well as independent of the impedance of the telephone set.
A problem with line feed circuits is the inherent internal impedance of such feed circuits. Any longitudinal sixty hertz signals induced in the subscriber loop from adjacent power lines appear as a voltage across the line feed impedance, causing audible interference with voice transmission. Capacitor bypasses for these audible components require a very large capacitor to accommodate the low frequency sixty hertz signal.